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Abstract

We perform a systematic study of the resonant transmission of visible and near-infrared (NIR) light through a single subwavelength slit in a gold film when the parameters defining the structure are varied. We further examine the optical properties of a related nanostructure, a cross with subwavelength sized features. Focused ion beam (FIB) milling was used to fabricate nanoslits and crosses with linewidths ranging from 26 nm to 85 nm. The dimensions of the structure are found to affect strongly the transmittance spectrum. For example, as the slit becomes narrower the resonance is observed to both sharpen and shift significantly. Our observations are in good agreement with our earlier numerical calculations on the optical properties of nanoslits.

Figures (8)

Scanning electron micrographs of nanoslits fabricated in gold film by focused ion beam milling: (a) 33 nm wide slit and (b) 31 nm linewidth cross. The inset in (a) displays a section of the slit at higher magnification. The angle θ in (a) and (b) defines the direction of polarization of the incident field.

Experimental setup used to study the nanoslit structures. Light from a tungsten halogen lamp (L) is passed through long-wave pass filter (LP), polarized with a polarizer (POL) and focused on the sample (S). The transmitted light is collected with a microscope objective (MO) and imaged onto either a charge coupled device camera (CCD) or coupled into a multi-mode optical fiber (MMF) that directs the light into a grating spectrometer (SM) equipped with detectors for the visible (VIS) and near-infrared (IR) spectral regions. An iris diaphragm (ID) is used to select only light emerging from one slit.

Normalized transmission spectra of nanoslits of different widths milled in a (193±2) nm thick gold film: 26 nm (black open squares), 35 nm (red open circles), 73 nm (green open triangles), and 85 nm (blue solid triangles) wide slits. The spectrum for a 26 nm wide slit milled in a (270±5) nm thick film is plotted with magenta open diamonds. The spectra have been normalized with the slit width in nanometers. The vertical line indicates the position of the resonance of the most narrow slit of the 193 nm thick film. The polarization of the incident light is orthogonal to the slit (θ=0°, see Fig. 1).

(a) Theoretically calculated resonance wavelength λres as a function of waveguide core thickness d. (b) Phase of the reflection coefficient of the waveguide ends with permittivities 1 (black continuous curve) and 2.25 (blue dashed curve) as a function of waveguide core thickness at the corresponding resonance wavelength. (c) The effective index of the mode neff and the normalized imaginary part of the propagation constant keff as a function of d at the corresponding resonance wavelength. (d) The dependence on d of the absolute value of the reflection coefficients r1 and r2 of the waveguide ends with permittivities 1 (black continuous curve) and 2.25 (blue dashed curve) at the corresponding resonance wavelength. In all calculations the length of the waveguide was t=193 nm and the medium in the waveguide had a permittivity of 1.

Transmission spectra of ≈30 nm wide slit and cross structures in 193 nm thick gold film as a function of the polarization direction θ (see Fig. 1) of the incident field: (a) spectral transmittance of a 25 µm long and 33 nm wide slit as a function of θ, (b) transmission spectrum of the slit for θ=4°, (c) spectral transmittance of a cross composed of two 25 µm long and 31 nm wide slits as a function of θ, and (d) transmission spectrum of the cross for θ=4°. The spectra have been normalized by the maximum transmittance of the cross. In (a) and (c) the vertical line is along the cross section θ=4°.

Polar plot of the maximum transmittance of the cross and slit structures as a function of the polarization direction θ of the incident field. The figure shows the transmittance along the horizontal line in Figs. 6(a) and 6(c) for the cross (black points) and the slit (blue circles), respectively. The solid line is a cos2θ fit to the data for the slit. The data point for θ=-6° was the start point and was measured a second time after completing a full circle. The data was normalized with the peak transmittance of the cross.

Change in transmission spectrum when a drop of 1,9-nonanedithiol is applied to the sample: transmission spectrum of a 26 nm wide slit milled in a 270 nm thick gold film for air (black open squares) and 1,9-nonanedithiol (red open circles) in the slit.